1. Field of the Invention
This invention relates generally to protective relays and more specifically to protective relays for pilot protection of electrical power lines.
2. Description of the Prior Art
Electrical transmission lines and power generating equipment must be protected against insulation faults and consequent short circuits, which could cause collapse of the power system, serious and expensive apparatus damage, and personal injury. For instance, such a fault condition is caused by lightning-induced flashover from a transmission line to ground or between adjacent transmission line conductors. Under such a fault condition, line currents can increase to several times the normal value thereby causing loss of synchronism among generators and damaging or destroying both the transmission line and the attached equipment. To avoid equipment damage and collapse of the entire power system, faulted apparatus on the main transmission line must be isolated from the network in the range of 0.1 to 0.5 seconds. The isolation time limit must allow for the operation of large circuit breakers, interrupting up to 80,000 A, and the completion of back-up operations if these primary protective devices fail to function properly. To allow sufficient time for circuit interruption, location of the fault must be determined in approximately 8 ms to 20 ms. It is the function of the protective relays, which monitor ac voltages and currents, to locate line faults and initiate isolation via tripping of appropriate circuit breakers. These faults are located by determining the relationship of the ac voltages and currents.
One type of protective relay system for transmission lines employs a protective relay apparatus at each transmission line terminal. Using the ac voltages and currents, each protective relay operates independently to determine if a fault has occurred on the intervening protective line segment. This type of protective relay system usually provides high-speed simultaneous protection for only 70% to 80% of the faults in the middle section of the protected line. For faults occurring near the ends of the line, the protective relay nearest the fault clears the line at high speed. The relay remote from the fault, however, clears the line with a time delay unless system conditions provide high speed sequential tripping.
In another type of protective relay system, known as pilot relaying, the protective relays at each end of the transmission line communicate with each other via a bidirectional communication link. The relays at each line terminal compare fault location information to quickly determine if the fault is on the intervening line segment. If the comparison indicates that the detected fault is internal, i.e., between the two protective relays, the intervening transmission-line section is isolated by tripping the circuit breakers at the line terminals. If the comparison indicates the fault is not between the two protective relays, the circuit breakers remain closed and the line operative. Pilot protection provides high speed protection for 100% of the protected line independent of system conditions. The communications channel linking the two relays can employ a modulated carrier signal transmitted over a power line phase conductor or a microwave system, or audio tones propagated on a leased telephone line or a dedicated pilot wire.
There are several advantages to high speed simultaneous clearing at all terminals for all line faults, including: reduced possibility of line damage and conductor burn-down; improved transient stability of the power system, and provisions for use of high speed reclosing, which if successful, improves transient stability, minimizes outage time, and improves voltage conditions on portions of the system load.
There are many different types of pilot relaying; the present invention relates to a blocking pilot relaying system. In the blocking system, the communications channel linking the protective relays at opposite ends of the protected line segment is used only to prevent one or more of those protective relays from tripping on an external fault. A channel signal is not required for internal faults, i.e., tripping occurs in the absence of a channel signal. Therefore, when a channel signal is present, the pilot protective relays are blocked from tripping.
To detect a fault with a pilot relaying system, either directional comparison relays or phase comparison relays are used. In the directional comparison scheme, fault detecting relays compare the direction of power flow at the two line terminals. Power flow into the line at both terminals indicates an internal fault, and the circuit breakers at each terminal of the line are tripped. If the power flows into the line at one terminal and out at the other, the fault is external and the line remains in service.
The phase comparison scheme generally uses overcurrent fault detecting relays to compare, via the channel, the relative phase of the currents at the terminals. If the currents at the terminals are relatively in-phase an internal fault is indicated, and the circuit breakers at both terminals of the line are tripped. If the currents at the terminals are relatively 180.degree. out-of-phase, an external fault (or through load current) is indicated and the breakers are not tripped.
Although the blocking schemes discussed briefly above are well-known in the prior art and have been used successfully for many years, they require specialized logic to suppress undesired trips that may result from the coordinated response of the protective relays to changes in fault character, e.g., clearing of an external fault. The digital logic and computer-based embodiments of the present invention take advantage of distinctive digital logic and software approaches to achieve more straightforward and easily coordinated behavior of the protective relays during a change in fault character. The present invention also reduces problems associated with coordination of relay operating times at the two terminals at the time of fault inception. The logic of the present invention can also be applied to conventional relays.